Changes in base composition bias of nuclear and mitochondrial genes in lice (Insecta: Psocodea)
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Changes in base composition bias of nuclear and mitochondrial genes in lice (Insecta: Psocodea) Kazunori Yoshizawa • Kevin P. Johnson
Received: 21 May 2013 / Accepted: 5 November 2013 / Published online: 14 November 2013 Ó Springer Science+Business Media Dordrecht 2013
Abstract While it is well known that changes in the general processes of molecular evolution have occurred on a variety of timescales, the mechanisms underlying these changes are less well understood. Parasitic lice (‘‘Phthiraptera’’) and their close relatives (infraorder Nanopsocetae of the insect order Psocodea) are a group of insects well known for their unusual features of molecular evolution. We examined changes in base composition across parasitic lice and bark lice. We identified substantial differences in percent GC content between the clade comprising parasitic lice plus closely related bark lice (=Nanopsocetae) versus all other bark lice. These changes occurred for both nuclear and mitochondrial protein coding and ribosomal RNA genes, often in the same direction. To evaluate whether correlations in base composition change also occurred within lineages, we used phylogenetically controlled comparisons, and in this case few significant correlations were identified. Examining more constrained sites (first/ second codon positions and rRNA) revealed that, in comparison to the other bark lice, the GC content of parasitic lice and close relatives tended towards 50 % either up from less than 50 % GC or down from greater than 50 % GC. In contrast, less constrained sites (third codon positions) in Electronic supplementary material The online version of this article (doi:10.1007/s10709-013-9748-z) contains supplementary material, which is available to authorized users. K. Yoshizawa (&) Systematic Entomology, Faculty of Agriculture, Hokkaido University, Sapporo 060-8589, Japan e-mail: [email protected] K. P. Johnson Illinois Natural History Survey, University of Illinois, Champaign, IL 61820, USA e-mail: [email protected]
both nuclear and mitochondrial genes showed less of a consistent change of base composition in parasitic lice and very close relatives. We conclude that relaxed selection on this group of insects is a potential explanation of the change in base composition for both mitochondrial and nuclear genes, which could lead to nucleotide frequencies closer to random expectation (i.e., 50 % GC) in the absence of any mutation bias. Evidence suggests this relaxed selection arose once in the non-parasitic common ancestor of Phthiraptera ? Nanopsocetae and is not directly related to the evolution of the parasitism in lice. Keywords Molecular evolution GC content Relaxed selection Slightly deleterious mutation
Introduction Understanding variation in the process of DNA substitution across lineages is fundamental to understanding the factors that influence molecular evolution. For strictly neutral mutations, the DNA substitution rate is predicted to be directly proportional to mutation rate, independent of population size (Kimura
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